The present invention generally relates to a copper foil for a printed circuit board and a copper clad laminate for a printed circuit board having superior chemical resistivity and adhesiveness, and in particular relates to a copper foil for a printed circuit board comprising a layer including nickel, zinc, a compound of nickel and zinc (hereinafter referred to a “nickel-zinc layer”) at least on an adherend surface with a resin of a copper foil, and a chromate film layer on the nickel zinc layer, and also comprising a silane coupling agent layer as needed, as well as to a copper clad laminate for a printed circuit board produced by using the foregoing copper foil.
A semiconductor package substrate is one type of printed circuit board, and is a printed circuit board that is used for mounting a semiconductor IC chip and other semiconductor devices. Since circuits that are formed on the semiconductor package substrate are finer than those formed on a standard printed circuit board, a resin base material that is different from a standard printed circuit board is used for the substrate material.
A semiconductor package substrate is generally manufactured according to the following processes. Foremost, a copper foil is laminated and bonded to a base material such as synthetic resin under high temperature and high pressure. This is referred to as a copper clad laminate or simply as a laminate. Subsequently, in order to form the intended conductive circuit on the laminate, a pattern such as resin with anti-etching properties is used to print a pattern that is equivalent to a circuit on the copper foil. The unwanted part of the exposed copper foil is eliminated by way of etching processing.
After the etching, the printed part is eliminated and a conductive circuit is formed on the substrate. Prescribed devices are ultimately soldered on the formed conductive circuit in order to form various printed circuit boards for electronic devices. This is ultimately bonded with a resist or buildup resin substrate.
Generally speaking, the quality demand of the copper foil for a printed circuit board differs with the adherend surface to be bonded with the resin base material (so-called roughened surface) and the non-adherend surface (so-called gloss surface), and it is necessary to simultaneously satisfy both demands.
Requirements of the gloss surface are as follows: (1) favorable appearance and no oxidative discoloration during the storage thereof, (2) favorable solder wettability, (3) no oxidative discoloration during high-temperature heating, and (4) favorable adhesion with the resist.
Meanwhile, main requirements of the roughened surface are as follows: (1) no oxidative discoloration during the storage thereof, (2) peel strength with the base material is sufficient even after high-temperature heating, wet processing, soldering, chemical treatment and the like, and (3) no so-called lamination contamination that occurs in etching after the lamination with the base material.
Moreover, pursuant to the finer circuit printed patterns in recent years, the lower degree of roughness of the copper foil surface is being demanded.
Further, with personal computers and electronic devices of mobile communication and the like, the higher frequency of electrical signals is progressing to accommodate faster speed and greater capacity of communication, and printed circuit boards and copper foils that are compatible with the foregoing demands are being sought. When the frequency of electrical signals becomes 1 GHz or greater, the influence of the skin effect where the currently flows only on the surface of the conductor becomes prominent, and the influence of the increase in impedance caused by changes in the current transmission path due to the irregularities of the surface can no longer be ignored. From this point also, the surface roughness of the copper foil is desirably low.
In order to meet the foregoing demands, numerous surface treatment methods have been proposed for use in a copper foil for a printed circuit board.
Although the surface treatment method differs with a rolled copper foil and an electrolytic copper foil, an example of a surface treatment method of an electrolytic copper foil is shown below.
Foremost, in order to increase the bond strength (peel strength) of the copper and the resin base material, generally speaking, particles formed from copper and copper oxide are provided on the copper foil surface (roughening treatment), and a heat-resistant layer (barrier layer) formed of brass, zinc or the like is formed to yield heat resistant properties.
Then, in order to prevent the surface oxidation and the like during the transportation or storage, corrosion prevention treatment such as chromate treatment based on immersion or electrolytic zinc, electrolytic zinc chromate treatment or the like is ultimately performed to obtain a product.
Among the above, in particular, the surface treatment method of forming a heat-resistant layer is a major key in deciding the surface properties of the copper foil. Thus, as the metal or alloy forming the heat-resistant layer, numerous copper foils forming a film layer of Zn, Cu—Ni alloy, Cu—Co alloy, Cu—Zn alloy and the like have been put into practical use (for instance, refer to Patent Document 1).
Among the above, a copper foil formed with a heat-resistant layer formed from Cu—Zn alloy (brass) is broadly used industrially since it yields superior characteristics such as no spots on the resin layer when used as a printed circuit board formed from epoxy resin or the like, and less deterioration in the peel strength of the copper foil after the printed circuit board is retained at a high temperature. Patent Document 2 provides a detailed description regarding the method of forming such a heat-resistant layer formed from brass.
In recent years, in the manufacturing processing of a printed circuit board; in particular a package substrate, the processing of roughening the copper foil gloss surface is being performed based on soft etching using mixed solution of sulfuric acid and hydrogen peroxide in order to improve the adhesion of the resist or buildup resin substrate and the gloss surface of the copper foil as the circuit side.
Nevertheless, when the foregoing soft etching using mixed solution of sulfuric acid and hydrogen peroxide is performed to the copper foil circuit gloss surface of a printed circuit board using a copper foil formed with a heat-resistant layer formed from brass, the corrosion (circuit corrosion) phenomenon of both ends (edges) of the previously formed circuit pattern will occur, and there is a problem in that the peel strength with the resin base material will deteriorate.
The circuit corrosion phenomenon is a phenomenon where the adhesive boundary layer between the copper foil circuit and the resin base material; that is, the circuit side surface where the heat-resistant layer formed from brass is exposed will erode due to mixed solution of sulfuric acid and hydrogen peroxide, whereby the roughened surface side in the vicinity of the edge portion of the circuit which should normally be a yellow color (caused by brass) becomes red, and the peel strength of the copper foil of that portion will deteriorate considerably. If this phenomenon occurs on the entire surface of the circuit pattern, the circuit pattern will peel from the base material and cause a major problem.    [Patent Document 1] Japanese Patent Application (Kokoku) Publication No. S51-35711    [Patent Document 2] Japanese Patent Application (Kokoku) Publication No. S 54-6701